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Maximum velocity in tube side

Maximum velocity in tube side

Maximum velocity in tube side

Dear colleagues,

whilst designing a BEM heat exchanger for warming up natural (tube) gas with steam (shell), I came up with a tube velocity of 23,48 m/s (77 ft/s). This was obtained with a shell diameter of 10".Tube count =51 with 373 " tubes. material is carbon steel.
The density at process conditions of GN is 17,45 kg/m3 so it results in a Rho *V2 value of 9616 kg/(m·s2)or 6460 (lb/ft s2)
The pressure drop is 0,179 kg/cm2 or 2,5 PSI.

If I use a smaller shell size, say 8", the dens·V2 value increases to 28569 kg/(m·s2)or 19193 lb/ft s2).Velocity increases to 41 m/s or 135 ft/s. The pressure drop increases up to 0,4 kg/cm2 or 5,7 PSI but this is no a problem in my system.

I was checking TEMA 9th Ed. and it says in RCB 4.63 Tube Side:

"Consideration shall be given to the need for special devices to prevent erosion of the tube ends under the following conditions:
-Use of an axial inlet nozzle.
-Liquid pv2 is in excess of 6000 (8928), where V is the linear velocity in feet per
second (meter per second), and p is its density in pounds per cubic foot (kilograms per cubic meter).

With the 8" shell size I am well above the limit of 8928 SI or 6000 (english units) and with the 10" I am approximately at the limit.

However, a couple of things intrigue me:

a) TEMA talks of LIQUID pv2 is in excess of. In my case, the fluid is not liquid but gas. Is it still applicable for gases?
b) Are these general recommendations and the 10" design is correct or the number is rigorous and I must never exceed the 6000 or 8928 figure.

I would really aprreciate your comments.

RE: Maximum velocity in tube side

The TEMA quantity is also good for gases. I would not exceed the 6000 by much.

On www.gulleyassociates.com in the pressure drop section, there is an equation for the maximum velocity inside tubes. In this case it gives a maximum value of 6400 where velocity is in ft/sec. and density is in lb/ft3.

RE: Maximum velocity in tube side

Agree, 23m/sec is usually too high; there may also be problems with tube bundle vibration. It may be marginally acceptable if you have completely corrosion resistant tube material, if mechanical failure risk due to bundle vibration can be designed out.

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